See your other thread - you cannot copy/paste fields.
You must use CTRL+F9 for each set of fields brackets.

As Graham says, you can't simply copy & paste - you have to use proper field braces (ie '{ }'), which you insert in pairs via
Ctrl-F9. And be careful to keep the same spacing. In:
{IF{MERGEFIELD "CSCS_Test_DateTime" \@ YYYYMMDD}> {DATE \@ YYYYMMDD}

make sure to delete the line feeds and '> ' character strings, so that there is a single space separating the fields.

Probably the easiest way to set the field up is to:
1. insert three copies of your basic mergefield, thus:
«CSCS_Test_DateTime» «CSCS_Test_DateTime» «CSCS_Test_DateTime»
2. Select each field in turn, press Shift-F9 to expose its code and edit it as shown above, adding the formatting as originally
described, then pressing F9 to update the field before moving on to the next one
3. Insert a DATE field between the first pair of mergefields, via Ctrl-F9 to create a pair of field braces, then typing 'DATE' thus:
«CSCS_Test_DateTime» {DATE} «CSCS_Test_DateTime» «CSCS_Test_DateTime»
4. Select all the fields and press Ctrl-F9 to wrap all of them in a single field, thus:
{«CSCS_Test_DateTime» {DATE} «CSCS_Test_DateTime» «CSCS_Test_DateTime»}
5. Add the IF test, making sure to preserve the inter-field spacing, thus:
{IF«CSCS_Test_DateTime»>{DATE} «CSCS_Test_DateTime» «CSCS_Test_DateTime»}
Note that there are two spaces.
6. Select the whole lot and press F9 to update.
7. Run your mailmerge.

Cheers